Speed sensors, or detectors, of various types are well known. In recent years the application of speed detectors to automotive display and automotive control functions has stimulated increased demands on and sophistication of, those sensors. In one common configuration, the speed sensor is placed in non-contacting relation with a member having reference points or teeth, the relative motion therebetween is detected by the sensor and associated circuitry provides at least timing reference signals and may ultimately provide an indication of the relevant speed. Usually, the sensor is fixedly positioned and the moving member is a toothed wheel, or so called "tone wheel", which rotates at a speed which is a known function of the speed to be measured.
One often used class of speed sensors employs one or more sense coils positioned in proximity with the rotating teeth of a tone wheel for exploiting either magnetic or eddy current effects depending on whether or not magnetic materials are present in the teeth.
One sensor of the aforementioned general type is disclosed in Peter W. Hammond U.S. Pat. No. 3,716,787. In that reference, there is disclosed a speed monitoring device having a phase measurement circuit which is utilized to measure the changes in the inductance of a sensor coil as affected by the proximity of magnetically distinct pins on a tone wheel. The relative increase and decrease in spacing between tone wheel pins, or teeth, and the spaces between such teeth with respect to the sensor coil serves to vary the impedance or inductance of that coil in a tuned circuit in which it is included. Such variation in the impedance effects a circuit phase-shift with respect to a predetermined reference signal of voltage and current in the circuit to indicate movement of the wheel and a measurement of the speed of movement in a measured time interval.
Another Said Sapir U.S. Pat. No. 3,750,128 discloses a pulse generator which produces output pulses at a pulse repetition frequency directly proportional to the wheel velocity. The output pulses may be used in a conventional anti-skid braking system. The pulse generator is operative even at low velocities since it employs an oscillator-energized variable reluctance transformer.
Yet another device for sensing speed is disclosed in Gerald O'Callaghan U.S. Pat. No. 3,728,565. That reference discloses first and second spaced-apart stator windings positioned near a rotating tone wheel. Respective sinusoidal voltages are induced in each of the stator windings and the magnitude of those voltages is indicative of the speed.
With respect to the U.S. Pat. No. 3,728,565 reference, it will be appreciated that the system is dependent upon the speed of rotation of the tone wheel for an amplitude signal to provide a corresponding speed signal. Such systems are inherently limited, particularly with respect to low speeds of operation and/or variations in the signal magnitude occasioned by other than speed alone. It is also desirable that the sensor operate over a relatively wide gap between it and the tone wheel, however, amplitude-dependent systems exhibit characteristic weaknesses in that regard. Though the U.S. Pat. Nos. 3,716,787 and 3,750,128 references are not dependent upon some minimum speed of the tone wheel for operability since they rely upon phase-shift techniques, they do possess other limitations. For instance, with respect to U.S. Pat. No. 3,716,787, the phase shift in the circuit containing the sensing coil is determined relative to a fixed reference signal from the oscillator which drives the sensing coil circuit. Since the reference signal has a fixed frequency, the circuit containing the sensing coil is capable of only a limited relative phase shift. Such limitation generally requires a relatively strong interaction between the tone wheel and the coil and circuitry to provide a desired response. In U.S. Pat. No. 3,750,128 a transformer interacts with the tone wheel and with a rectangular wave generator to create the requisite phase shifts.
Accordingly, it is a principal object of the invention to provide an improved speed detector apparatus which is operative at all speeds.
It is a further object of the present invention to provide an improved speed detector apparatus which is operative over a relatively wide operating gap between the sensor and the tone wheel, particularly with a substantially constant signal amplitude. Included within this object is the provision of such apparatus for use with a tone wheel having metal teeth of either a magnetic or a non-magnetic material.
It is a still further object of the invention to provide an improved speed detector apparatus in which the associated circuitry is of a digital character.
It is a yet further object of the invention to provide an improved speed detector apparatus which is accurate, relatively compact and susceptible of economical manufacture.